Okay, let me ask for another opinion. I think the HRT's headphone amp sounds much better than the Cayin in terms of detail and separation, and listening to uptempo or complex music, especially hi-res, keeps me (sometimes for the worse, if I'm doing more than just listening to music) hypnotized following every little nuance. It also has better imaging, which I really prize, than the Cayin and brings out the Sennstage much more. The Cayin is a great amp, don't get me wrong, but the sensitivity of these phones with the very prominent bass boost renders that feature a big blob of mud, and despite the treble fatigue issues with the HRT, the bass is actually much more well controlled than sending it through the Cayin...it's just the low mids that really suffer.
So, Cayin aside for a second, and given that I listen to a lot of Lou Reed, Bob Dylan, Tom Waits, Joanna Newsom, Ryan Adams, Wilco, etc....with just a smattering of jazz and Bob Wills (awwwww hawwwww) and John Prine and Loretta....would the 600's reputed "faster" sound be a better compromise between the 598s and the "laid-back" 650s IF I were to trade the 598s for a pair of ATH-M50x's for a more portable set and a better pairing for the HRT's headphone output? Basically giving me even more of a contrast between what I have and the 6xx and allowing me to choose the headphone (perhaps) better suited to my music, the 600....and that's a big perhaps....
And I'm still set on a Crack or Ember. Maaaaybe Valhalla 2 or Lyr 2 whne the prices drop, but DIY is its own special caveat to me.
W/R/T the 700s mate, anything that high is a bit more than I'm looking for at the moment. I'd like a couple of decent options when I go to grab a headphone.